A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, to manufacture integrated circuits (ICs). In such a case, a patterning device, for example, a mask or a reticle, can generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (for example, including part of, one, or several dies) on a substrate (for example, a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. Generally, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatuses include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In the lithographic apparatus, the radiation beam may cause thermal effects (e.g., thermal expansion) in the patterning device. The patterning device may comprise a base material, for example, fused silica, that is substantially transparent to radiation, for example, deep ultraviolet radiation, and may comprise a pattern made of a substantially non-transparent material, for example, chrome. The thermal effects may be due to absorption of radiation beam by non-transparent portions of the patterning device and may cause, for example, alignment errors and/or overlay errors in the patterns formed on the substrate. The radiation beam or convection from the heated patterning device may also heat the air between the patterning device and proximal lens elements. Such heated air between the pattering device and the lens elements may cause image distortion (such as overlay errors). To correct these errors due to thermal expansion of the patterning device and/or heating of the air between the pattering device and the lens elements, current lithographic apparatus may rely on correction systems. For example, such a correction system can include a reticle or wafer alignment system, a magnification correction system, a feed forward system for expansion prediction, a lens correction system, or a combination thereof. However, with the continuing trend towards scaling down of device dimensions, these correction systems may not provide the desired level of alignment and/or overlay accuracy that may be needed for the development of these scaled down devices.